1. Field of the Invention
The present invention relates to a gear ratio control system for a continuously variable transmission to be used in a vehicle.
2. Related Art
The continuously variable transmission is represented by a belt type continuously variable transmission which is provided with a primary pulley (or an input unit), a secondary pulley (or an output unit), and a belt.
Moreover, the feedback control is so made that the real primary speed of the primary pulley may coincide with the target primary speed.
In this case, the target primary speed is set according to a vehicle speed and a throttle opening on the basis of the control characteristics, as illustrated in FIG. 7.
With the throttle being fully closed (to 0%), for example, the target primary speed is so set, as illustrated in FIG. 7, that the gear ratio of the continuously variable transmission may be an overdrive (OD) according to the vehicle speed.
If the throttle is fully closed during a downhill running time and if the target primary speed is set so that the gear ratio of the continuously variable transmission may be the overdrive (OD), as illustrated in FIG. 7, however, a sufficient engine braking may not be established.
In order to establish the sufficient engine braking during the downhill running time, therefore, it is conceivable (as disclosed in Japanese Patent No. 2840233, for example) to make the gear ratio control of the continuously variable transmission so that the real acceleration of the vehicle may become the target acceleration set on the basis of the engine output characteristics.
During an ordinary running time such as during a flat road running time, generally, the gear ratio control of the continuously variable transmission is made such that the real primary speed of the primary pulley belonging to the continuously variable transmission may become the target primary speed. Therefore, it complicates the control and invites an increase in the program capacity to add, as in the aforementioned prior art, the gear ratio control of the continuously variable transmission, in which the rear acceleration of the vehicle becomes the target acceleration for the downhill running. At the control switching time, on the other hand, the gear ratio may abruptly change at the control switching time to make the driver feel a physical disorder and to deteriorate the drivability.
When the throttle is fully closed during the downhill running, on the other hand, the engine output characteristics may disperse to deviate the target acceleration from the intrinsic value.
In this case, the actual acceleration (or the real acceleration) may be controlled to a target acceleration shifted from the intrinsic target acceleration. This makes it difficult to make an accurate control so that the rear acceleration may be the intrinsic target acceleration.
The engine braking state of the vehicle running downhill depends on the preference of the driver. Some driver desires to run downhill promptly with no engine braking, and another driver desires to run slowly with an intense engine braking. If the target acceleration at the downhill running time is uniformly set according to the road slope and the vehicle speed, therefore, the preference of the driver is ignored to deteriorate the driving feel.
The invention has been conceived in view of the problems thus far described and has an object to provide a gear ratio control system for a continuously variable transmission, which can establish a sufficient engine braking at a downhill running time with neither complicating the control nor inviting an increase in the program capacity.
Another object of the invention is to provide a gear ratio control system for a continuously variable transmission, which is enabled to establish a stable engine braking feel by making an accurate gear ratio control without being influenced by the dispersion in the engine output characteristics with the fully closed throttle even while the vehicle is running downhill.
Still another object of the invention is to provide a gear ratio control system for a continuously variable transmission, which can control the engine braking state during the downhill running time in accordance with the preference of the driver.
According to a first aspect of the invention, there is provided a gear ratio control system for controlling gear ratios of a continuously variable transmission coupled to an engine, comprising: a setting element for setting a target speed of an input unit of the continuously variable transmission at a downhill running time on the basis of a target output of the engine; and a control element for controlling the gear ratios of the continuously variable transmission so that the rear speed of the input unit may become the target speed of the same.
According to a second aspect of the invention, on the other hand, the setting element includes: a decision element for deciding whether or not learning conditions are satisfied with a fully closed throttle; an updating element for updating a learning reference torque, if said decision element decides that the learning conditions are satisfied, on the basis of a reference torque or the engine torque corresponding to a real engine output and a learning reference torque set at the preceding time of satisfying the learning conditions; and a determining element for determining engine output characteristics on the basis of the learning reference torque updated by the updating element.
According to a third aspect of the invention, on the other hand, the setting element includes: a target acceleration setting element for setting a target acceleration according to a road slope so that the real acceleration may become the target acceleration; a properness deciding element for deciding the excess/deficiency of an engine braking; and a learning correcting element for learning and correcting the target acceleration on the basis of the decision of the properness deciding element.
According to a fourth aspect of the invention, on the other hand, the setting element includes: a target acceleration setting element for setting a target acceleration according to a road slope; a target output setting element for setting a target output of the engine in accordance with the target acceleration; and a target speed setting element for setting a target speed of the input unit of the continuously variable transmission in accordance with the target output.